1. Field of the Invention
The present invention relates to a waste heat boiler construction successive to a suspension smelting furnace, particularly a flash smelting furnace, in which construction the dust-bearing gases created in the suspension smelting furnace are prevented from directly flowing from the radiation section of the waste heat boiler to the convection section, in order to reduce the tendency to dust build-ups caused by the gases, and in order to advantageously utilize the total volume of the waste heat boiler as well as to extend the residence time. The invention also relates to a method for intensifying gas mixing and for extending the total residence time in the radiation section of the waste heat boiler.
2. Description of the Prior Art
Generally the waste heat boiler used after a suspension smelting furnace is a tunnel-type boiler operated with direct gas flow, which boiler is divided into two sections, the radiation section and the convection section. The purpose of the radiation section is to cool the gases so that the molten particles contained in the gas are solidified and their temperature decreases to below the particle sintering temperature before the gases are conducted to the convection section of the waste heat boiler. In the convection section, the end heat contained by the dust-bearing gases is recovered by means of a cooling tube bank.
However, in tunnel-type waste heat boiler constructions the high dust content of the gases created in suspension smelting often results in dust build-ups which hamper the operation of both the waste heat boiler and the whole suspension smelting process. The losses caused for the producers by possible interruptions in the suspension smelting process due to these difficulties are remarkable. The tendency to dust build-ups is favoured for instance by the following factors:
In the radiation section of the waste heat boiler, only the ceiling and the upper parts of the walls are in efficient use, and only when they are clean. Because a large share of the heat load is directed onto a small proportion of the boiler, it is difficult to keep the waste heat boiler clean. Further, the hot dust-bearing gases flow, partly uncooled, directly to the convection section of the boiler, in which case molten dust particles stick to the cooling tube bank, and the cooled particles are sintered. Moreover, the bottom part of the waste heat boiler works poorly as a radiation receiver, but allows a long residence time for part of the dust-bearing gases, thus creating conditions for a disadvantageous reaction where sulfur dioxide is oxidized into sulfur trioxide. At the gas scrubbing stage of a sulfuric acid plant, sulfur trioxide forms sulfuric acid, so-called scrubbing acid, which often is not far from hazardous waste.
The removal of dust build-ups from a waste heat boiler has been attempted in many different fashions; the cleaning of the boiler has been intensified by means of hammering devices--with positive results, but only in removing dust, not eliminating the cause of the problem. The disadvantages of too intensive hammering will soon be obvious: the working life of the waste heat boiler is shortened. In the radiation section of the waste heat boiler, there have also been constructed cooling panels parallel to the boiler, so that the gas can freely flow in between the said panels; these panels are known to work well, if they are correctly designed. Moreover, transversal cooling panels--i.e. transversal to the gas flowing direction--have been tested in the radiation section of the boiler. However, the experiences have been disappointing because of an active tendency to slag crust formation. Attempts have also been made to prevent the direct flowing of the gas along the ceiling of the radiation section of the waste heat boiler, by placing the convection section lower than the radiation section, so that the rear part of the ceiling of the radiation section is inclined downwards.
From the U.S. Pat. No. 4,530,311 there is known a waste heat boiler where the construction of the radiation section has been altered in order to eliminate the drawbacks of the above described constructions. In relation to the front end of the radiation section, the convection section of the waste heat boiler is located on an essentially lower level in order to prevent the dust-bearing gases from flowing directly along the ceiling of the radiation section. The ceiling of the radiation section is constructed so that the radiation section is stepwise lowered down to the level of the front end of the convection section, and state-of-the-art hammering devices are at the same time switched to the wall. Thus it is possible to make use of the bottom of the radiation section that in prior art embodiments remains inefficient. Morerover, in the ceiling of the radiation section, there are installed panels parallel to the gas flowing direction, so that in the compartments formed by the transversal walls of the radiation section, the panel of the successive compartment always divides the gas flows from the previous compartment more or less into two.